This invention relates to an A.C. motor drive apparatus for raising the torque of an A.C. motor such as an induction motor or synchronous motor.
It is well known in the art that variable frequency (VF) control and variable voltage-variable frequency (VVVF) control are available as control methods in a control apparatus for converting direct current into alternating current by means of an inverter circuit to drive an induction motor through use of the alternating current. With a control apparatus relying upon the VF control method, a primary frequency, which is the output of the inverter circuit, is varied in accordance with a speed command. With a control apparatus that operates based on the VVVF control method, the amplitude of the primary voltage also is varied in proportion to the change in primary frequency, with the output torque being held constant. These control apparatuses deal with the voltage and current applied to the induction motor in terms of amplitude and frequency, but both of them effect control based on mean values. It is not possible, therefore, to achieve fine control with good response. Accordingly, in order to improve upon this disadvantage, a so-called "vector control apparatus" has recently been developed and put into practical use. With the vector control apparatus, a pulse-width control method is employed to control the momentary value of the stator current of an induction motor, enabling torque generation similar to that seen in a shunt-wound D.C. machine. The vector control method applied to induction motors is based on the torque generating principle of a shunt-wound D.C. machine and controls the momentary value of a stator current to generate a torque in the same manner as said D.C. machine.
The torque generating mechanism of a shunt-wound D.C. machine is such that a current switching operation is effected by a commutator in order that the magnetomotive force of an armature current I.sub.a will lie perpendicular to the main magnetic flux .phi. at all times. The generated torque Ta is expressed by the following equation, the torque T.sub.a being proportional to the armature current I.sub.a if the main magnetic flux .phi. is constant: EQU T.sub.a =k.multidot.I.sub.a .multidot..phi. (1)
In order to apply the foregoing relation to an induction motor, correspondence is established between .phi. and the magnetic flux vector .phi..sub.2 of a rotor, and between I.sub.a and a secondary current vector I.sub.2. Accordingly, to drive an induction motor in accordance with a principle resembling the generation of a torque by means of a shunt-wound D.C. machine, control should be effected in such a manner that the relation between the rotor flux vector .phi..sub.2 and the secondary current I.sub.2 is a perpendicularly intersecting one. The generated torque T.sub.a, neglecting secondary leakage reactance, is expressed by: EQU T.sub.a =k I.sub.2 .phi..sub.2 .div.k I.sub.2 .phi..sub.m ( 2)
(where .phi..sub.m is the main magnetic flux arising from an excitation current I.sub.o).
Let us consider the stator current applied to the stator windings in the case of, say, a two-phase induction motor. Let the A-B axes represent the static coordinate system of the stator, I.sub.1 the stator current (primary current), I.sub.o an excitation current component, and I.sub.2 a secondary current. Also, let I.sub.1a, I.sub.1b denote the A- and B-axis components of the stator current I.sub.1, namely the A-phase stator current and B-phase stator current, respectively. If we assume that the main flux .phi..sub.m is rotating with respect to the static coordinate system of the stator at an angle of rotation .phi. (.phi.=.omega.t if the angular velocity is .omega.), then the A-phase stator current I.sub.1a and B-phase stator current I.sub.1b will be expressed by the respective equations: EQU I.sub.1a =I.sub.o cos .phi.-I.sub.2 sin .phi. (3) EQU I.sub.1b =I.sub.o sin .phi.+I.sub.2 cos .phi. (4)
Thus, in accordance with the control apparatus that operates based on the vector control method, the A-phase and B-phase stator currents I.sub.1a, I.sub.1b indicated by Eqs. (3), (4) are generated and applied to the stator windings (primary windings) to drive the induction motor. When the load changes, only the secondary current I.sub.2 is increased or decreased accordingly, with the excitation current I.sub.o being held constant.
Driving the aforementioned A.C. motor is accomplished by obtaining, from a current command circuit, a current command amplitude based on an arithmetic difference between a speed command signal and an actual speed signal from a speed feedback loop that indicates the actual speed of the A.C. motor, generating a current command in each phase based on the current command amplitude, this being achieved by means of a phase command generating circuit, obtaining from a difference output circuit a difference current between each phase of the current command and respective phase currents actually applied to the A.C. motor, these phase currents being obtained from a current feedback loop, amplifying each difference current by an amplifying circuit and driving the A.C. motor by the output of the amplifying circuit. The current command in each phase is generated with the amplitude of the sinusoidal signal corresponding to each phase serving as a current command amplitude. The current command in each phase is obtained by multiplying the corresponding sinusoidal signal by the amplitude of the current command. The current command in each phase produced in this manner is delivered through a current amplifier. Since the current amplifier has a saturation characteristic, the amplifier will saturate when the amplitude of the corresponding sinusoidal signal is large, giving rise to distortion of the sinusoidal signal waveshape. The current command in each phase will therefore include a wave component other than the fundamental harmonic (wave) component, which is sinusoidal. In other words, the current command in each phase will come to include a higher harmonic component. Because the higher harmonic component is generated independently of the fundamental harmonic, it plays no part in the A.C. motor torque, and an increase in the higher harmonic component will result in a drop in the voltage applied to the motor. Accordingly, for a constant A.C. input voltage, the torque-rotational speed characteristic of the A.C. motor will take on the appearance shown in b of FIG. 1. In FIG. 1, b indicates the characteristic that results with inclusion of the higher harmonic component arising from saturation of the current amplifier, whereas a shows a characteristic in which only the fundamental harmonic is included. A defect in the prior art, therefore, is that the decline in voltage applied to the A.C. motor causes gives rise to a lower torque, as depicted by characteristic b, resulting in poorer stability with respect to an external load.